I COULD say something in answer to most of Delpino's ingenious criticisms,1 but I will confine myself to one point; remarking, however, that I cannot
see the force of his objection to the belief that the gemmules have the
power of self-division. The analogy of the multiplication within the body
of an infected person of the contagious atoms of small-pox or scarlet-fever
still seems to me fair. The criticism which at first struck me as much the
most forcible, relates to the re-growth of an amputated limb or other organ.
I have assumed, from what we know of the ordinary laws of growth, that gemmules
become developed only when united with the nascent or very young cells which
precede them in the due order of successive development. Now, Delpino remarks
that when the limb of a mature salamander is cut off, nascent cells would
not exist on the stump, but only mature ones; consequently, as he urges,
the gemmules, according to the hypothesis, could not be there developed
and form a new limb. But have we any reason for assuming that with animals the same cells or
organic units endure throughout life and are not replaced by new cells?
All physiologists admit that the whole frame is being constantly renovated.
The bones, which it might be thought would be least capable of renewal,
undergo before maturity great changes in form and size and in the diameter
of the medullary canal; even after maturity their shape does not remain
the same. If, then, it be admitted that the tissues throughout the body
are constantly being renovated by means of the old cells giving birth through
proliferation or division to new cells, the difficulty of the gemmules being
developed at any point at which a limb may be cut off, disappears; for some
nascent cells would exist there; and we must bear in mind that gemmules
may possibly become developed in union with cells which are not strictly
the proper ones, as shown in the extreme case of nails occasionally growing
on the stumps of the amputated fingers of man. It is probable that the renovation
of the tissues by the formation of new cells becomes less and less in old
age, and so does the power of regrowth. The curious fact that insects which
live but a short time after their final metamorphosis, have singularly little
power of regrowth, may be partly explained by the deficiency in their tissues
of nascent cells.

In the case of plants, the cells composing the tissues endure for life,
and are not renewed. The woody cells in the trunk of a tree originally existed
as delicate cells in the young shoot. Now, it is remarkable, considering
how low plants stand in the scale of life, what little power of re-growth
most of their organs or parts possess, not withstanding their high vitality.
Cut off a young shoot, and the next bud below will form a new shoot; cut
off an old stem, and hundreds of adventitious buds will be formed. No one,
I believe, ever saw a mutilated leaf repaired in the same manner as the
limb of a salamander; yet on the cut margins of certain leaves innumerable
buds will be quickly developed. In the bark, on the other hand, new cells
must be continually forming, for the outer layers are continually being
thrown off, and bark can certainly reproduce itself on a decorticated surface
(see Professor Oliver in Transactions of Tyneside Club, vol. iii.
1855, p. 67),2 from an isolated point not connected with the surrounding
uninjured bark; so that this case is fairly analogous with that of the re-growth
of an amputated limb.

I will add only one other remark. Delpino insists that, according to the
hypothesis, the same cells must throw off gemmules at many successive periods
of their development. But I can see no such necessity. Certain epidermic
cells, for instance, form horn; but their conversion into this substance
is apparently due to the chemical nature of the contents of the cells; and
of the changes consequent on the absorption of certain elements. An atom
or gemmule thrown off at an early period from one of these cells would,
when properly nourished, first form a cell like the parent cell, and ultimately
be converted into horn, without the necessity of throwing off successive
gemmules. As, however, a cell is a complex structure, with its investing
membrane, nucleus, and nucleolus, a gemmule, as Mr. G. H. Lewes3 has remarked
in his interesting discussion on this subject (Fortnightly Review, Nov.
1, 1868, p. 508), must, perhaps, be a compound one, so as to reproduce all
the parts. The attack by Delpino on the weak point of the hypothesis, namely,
that it requires the support of several subordinate hypotheses, is perfectly
just, though perhaps pushed to an extreme. But I can speak from recent experience,
that he who has to consider complex cases of inheritance, as limited either
separately or conjointly by sex, age, and season, with the inherited characters
themselves and the form of inheritance liable to change from crossing and
variability, will be able to disentangle the phenomena much more clearly,
if he admits for the time our hypothesis with all its imperfections. He
will then have fixed in his mind that transmission and development are quite
distinct phenomena,—the gemmules being thrown off from all parts of the
organization, and transmitted from both parents to both sexes at the earliest
age,—their development alone depending on the nature of the nascent tissues
of the individual, whether permanently or temporarily modified by sex, age,
season of the year, or other conditions.

CHARLES DARWIN.

Down, Beckenham, Kent.

1 Professor Delpino's opinions, as translated from the Rivista Contemporanea Nazionale Italiana, having appeared very fully in our columns, we have requested Mr. Darwin to reply to them. To this Mr. Darwin has complied, and we believe that the natural history world will be interested in reading the following remarks.—Ed. S.O.

1 Giacomo Guiseppe Federico Delpino (1833-1905), Italian botanist and professor of Botany at Genoa and later at Naples. Darwin was responding to the criticisms of Delpino 1869a to Darwin's 'Provisional hypothesis of pangenesis', chapter 27 in Variation 2: 357-404. This letter was later published in Italian with introductory remarks in Delpino 1869b. See Correspondence vol. 17.